diff --git a/lib/wbench/browser.rb b/lib/wbench/browser.rb
index 9ff27e1..2d27440 100644
--- a/lib/wbench/browser.rb
+++ b/lib/wbench/browser.rb
@@ -10,7 +10,13 @@ def initialize(url, options = {})
         selenium_options    = { :browser => browser, :http_client => http_client }
 
         if options[:user_agent]
-          add_selenium_args(selenium_options, "--user-agent='#{options[:user_agent]}'")
+          add_selenif browser == :firefox
+            profile = Selenium::WebDriver::Firefox::Profile.new
+            profile['general.useragent.override'] = options[:user_agent]
+            selenium_options[:profile] = profile
+          else
+            add_selenium_args(selenium_options, "--user-agent='#{options[:user_agent]}'")
+          end
         end
 
         SeleniumDriver.new(app, selenium_options)